Polyethylene is widely used for industrial applications in which a material having a high mechanical strength and a high resistance to thermooxidative degradation is required in order to ensure a long life even at elevated use temperatures. In addition, polyethylene has the particular advantages that it has good chemical resistance, it has a low intrinsic weight and it is a material which can easily be processed in the melt.
The use of bimodal or multimodal ethylene polymers in coating of steel pipes is known from EP-A-837915, EP 2 072 586, EP 2 072 587, EP 2 072 588 and EP 2 072 589. WO 2006/053741 discloses the use of a trimodal polyethylene composition having a density of 0.94 to 0.95 g/cm3 comprising 45 to 55 wt. % of a low molecular weight ethylene homopolymer A, from 30 to 40 wt.-% of a high molecular weight copolymer B and from 10 to 20 wt.-% of an ultra high molecular weight ethylene copolymer C for the coating of steel pipes. EP 1655334 reports the use of a polyethylene composition made of three fractions having varying molecular weight for the preparation of pipes, whereby the composition has a high eta747 of at least 350 kPas.
Polyethylene compositions having a significant fraction of ultra high molecular weight material have better processability than classic bimodal polyethylene compositions and have several advantages as to the mechanical properties such as environmental stress cracking resistance. However, processability of such polyethylenes still is limited at high throughput and the provision of adhesion to the usually used adhesive layer obtained by grafting with an acid grafting agent such as maleic acid or maleic anhydride is limited. Thus, there is the need for ultra high throughput coating compositions which have better processability and excellent adhesion to standard adhesion layers while retaining excellent levels of mechanical properties. Even further, the need for CO2 reduction asks for materials allowing less energy intensive production.